Computer Concepts and Terminology

Storage Devices

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Storage devices, such as
disk drives, store your documents (data files)
and programs (executable
files) when they are not currently in use for processing. Unlike
the contents of RAM, the data stored on these devices does not vanish
when power is turned off.

The major categories of storage devices are magnetic, solid
state, and optical.

Hard Disk Drive

A hard disk drive
contains disks made of metal and coated with a metal oxide that can be
magnetized. A tiny electromagnetic
read/write head on the end of a seek arm magnetizes tiny spots on the
disk to store data. Magnetic spots magnetized in one direction represent
a one; spots magnetized in the opposite direction represent a zero (OK,
I simplified things a little, but you get the idea). The same electromagnetic
head can later sense the magnetic fields of the spots as they pass underneath
the head, allowing the data to be read back from the disk.

Hard drives are rated by their storage
capacity, typically
tens or hundreds of gigabytes. They are also rated by how fast the disks
spin (in rpm, rotations per minute), which
is typically thousands of rpm. Another way to rate a hard disk is by
average access time (measured in milliseconds, ms),
which tells on average how long it would take the drive to retrieve any
bit of data from the disk. Typical seek times are around 6 ms.

The electronics that control the hard disk often incorporate
some cache memory. The drive reads in several sectors of data instead
of just one—that way, if the CPU happens to request those next
sectors, the drive can send them immediately without having to wait for
the disk to rotate back around again.

The controller electronics for a hard drive may be IDE,
or ATA, or SCSI,
or something else. Don’t worry about this detail here, but you do need
to get the right kind to go into your computer if you want to add additional
drives. You can also plug additional hard drives externally into the
USB or Firewire ports of a computer, if desired.

Floppy Diskette

In a floppy diskette the
disk is made of flexible mylar plastic coated with metal oxide that can
be magnetized. Floppy diskettes are 3.5" in size (older style floppy
diskettes for early PCs were 5.25").

A shutter protects the disk surface from dirt and fingerprints;
the shutter slides out of the way when the disk is inserted into the
drive so that the read/write heads can reach the disk.

A small plastic slider can be slid to unblock a hole in
the corner of the diskette to write-protect the disk (so data can’t
be accidentally erased).

High-density floppy diskettes
hold 1.44
MB. The access
time is much slower than for a hard disk, and they are somewhat
unreliable. Many new computers don’t have a floppy drive, but you can
purchase an external drive to plug in if you need to.

Zip Disk

A Zip disk is
similar in size to a floppy diskette, but thicker. It is basically a
“super floppy” but the higher construction tolerances and
smaller read/write heads allow the Zip disk to hold more data than a
floppy. The first Zip disks held 100 MB. Later Zip drives could read 250
MB Zips (in addition
to the old 100 MB disks). An even newer model Zip drive uses 750 MB disks.

Both Zip disk and floppy diskettes have the advantage of
being removable media. Data stored on these
disks can be removed and taken to other locations. Both Zips and floppies
can be formatted for either the PC or the Macintosh (Macs can read both
formats).

We had Zip drives in our previous PCs and Macs at UNM-LA,
but our newer computers don't use these, so you may never have to deal
with them.

Flash Drive

Zip Disk (250 MB)

A USB Flash Drive is
a portable solid state memory device that plugs into a USB port on your
computer. They have many other names (such as key drive, pocket drive,
thumb drive, pen drive). They have replaced floppy diskettes and Zip
disks at UNM-LA as our preferred means to carry files around. They work
on both Macs and PCs.

These small drives store data on flash
memory microchips (a
kind of EEPROM). Flash memory can be erased and re-written a limited
number of times (typically many thousands of times). Some units have
a write-protect switch.

The storage capacity varies, but anything from 16 MB to
over a gigabyte are available.

Flash Memory Cards

The same kind of flash memory
used in the USB flash drives above are is used in small memory cards
(a Secure Data SD card, and a Compact Flash card are shown on the right).
These cards are used by PDAs, digital cameras, MP3 music players,
and other digital devices. You can attach a flash memory card reader
to your computer to read and write data to these cards as well. These
memory cards (and other types not shown here) come in a variety of storage
capacities from tens of megabytes to over a gigabyte.

CD-ROM

Example Flash Memory cards (SD card left, Compact Flash card right)

A CD-ROM (Compact
Disc Read Only Memory) is an optical storage medium
that can hold about 670MB. “Optical” means that light is used to read
the data from the disk (it is not a magnetic medium). CD-ROMs are very
cheap to produce in large quantities, so most software is distributed
on CD-ROMs.

Data is stored on a CD-ROM as small pits
in the plastic of an inner layer, which is then aluminized and overcoated
with another clear layer. A laser beam inside
the CD-ROM drive is bounced off the disk and the sequences of pits
and not-pits (the reflectivity is different) is converted into the ones
and zeros of the data.

CD-ROM drives are rated by speed, such as 32x, which means
32 times faster than the first CD-ROM drives.

CD-R and CD-RW

CD-ROM disc

CD-R (Compact
Disc - Recordable) and CD-RW (Compact
Disk - ReWritable) are CDs that can be written
to (if your computer has a CD-RW drive).

The CD-R discs have a layer
of dye that is changed by a higher power laser in the drive to record
data (the low power reading laser does not change the data). The CD-R
can only have its data surface changed ONCE at each spot (although
you can write multiple sessions to one disk until it is full). After
that, it is read-only. CD-Rs can hold 700 MB of data.

The CD-RW discs contain a phase-change material that different
power laser beams can read, write, and erase, so these disks can be used
many times (but must be erased before re-writing).

DVD

DVD-ROM discs
(DVD = Digital Versatile Disc) are optical storage media similar to CD-ROMs,
but with a higher storage capacity. DVDs use smaller spots to record
data, and the disks can be dual-layer and double-sided, with each layer
holding
4.7 GB of data (so a dual-layer/double-sided
DVD can hold 18 GB of data).

Like CDs, DVDs also have recordable variants, although
there are still multiple formats (DVD-R and DVD+R) competing for dominance.
A single-layer DVD-R can hold 4.7 GB of
data (Dual Layer discs can hold twice as much).

DVD drive speeds are rated in terms of how many times faster
that the original DVD drives they are (a 6x DVD drive is 6 times faster)